Devices and formulations for pulmonary vaccination

Article (PDF Available)inExpert Opinion on Drug Delivery 10(10) · June 2013with129 Reads
DOI: 10.1517/17425247.2013.810622 · Source: PubMed
Abstract
Introduction: Pulmonary vaccination could be a promising alternative to vaccination by injection. Administration of a vaccine to the lungs does not require the use of needles, which reduces the number of trained healthcare workers needed, the risk of needle-stick injuries and needle waste. Besides a systemic immune response, pulmonary vaccination may also induce a mucosal immune response. Such a local response may increase the effectiveness of vaccination against airborne pathogens. Although this route of administration has been studied for decades, no pulmonary vaccine is commercially available yet, due to various challenges mostly intrinsic to pulmonary drug delivery and vaccine formulation. Areas covered: This review discusses the inhalation devices and formulation strategies that may be suitable for the pulmonary administration of vaccines. In addition, critical parameters are addressed, such as the target population, to help assessing whether pulmonary administration of a specific vaccine may be feasible and beneficial or not. Expert opinion: A combined approach of inhalation device and vaccine formulation development is essential. This should result in a system that can effectively be used by the target population and can be produced at low costs. Only then, this challenging administration route can be successfully applied to large-scale vaccination programs.

Figures

1. Introduction
2. Inhalation devices
3. Formulations
4. Conclusion
5. Expert opinion
Review
Devices and formulations for
pulmonary vaccination
Wouter F Tonnis
, Anne J Lexmond, Henderik W Frijlink,
Anne H de Boer & Wouter LJ Hinrichs
University of Groningen, Department of Pharmaceutical Technology and Biopharmacy, Groningen,
The Netherlands
Introduction: Pulmonary vaccination could be a promising alternative to vac-
cination by injection. Administration of a vaccine to the lungs does not
require the use of needles, which reduces the number of trained healthcare
workers needed, the risk of needle-stick injuries and needle waste. Besides a
systemic immune response, pulmonary vaccination may also induce a mucosal
immune response. Such a local response may increase the effectiveness of vac-
cination against airborne pathogens. Although this route of administration
has been studied for decades, no pulmonary vaccine is commercially available
yet, due to various challenges mostly intrinsic to pulmonary drug delivery and
vaccine formulation.
Areas covered: This review discusses the inhalation devices and formulation
strategies that may be suitable for the pulmonary administration of vaccines.
In addition, critical parameters are addressed, such as the target population,
to help assessing whether pulmonary administration of a specific vaccine
may be feasible and beneficial or not.
Expert opinion: A combined approach of inhalation device and vaccine for-
mulation development is essential. This should result in a system that can
effectively be used by the target population and can be produced at low
costs. Only then, this challenging administration route can be successfully
applied to large-scale vaccination programs.
Keywords: aerosol, dry powder inhaler, nebulizer, powder formulation, respiratory delivery,
vaccine
Expert Opin. Drug Deliv. (2013) 10(10):1383-1397
1. Introduction
Vaccination is the most effective and cost-effective way to prevent the dissemination
of infectious diseases. Therefore, it can be considered as one of the most successful
medical interventions ever. During vaccination, a weakened pathogen is adminis-
tered to the body, thereby inducing an immune response without causin g ser ious
symptoms of disease. Vaccines generally consist of live attenuated pathogens, whole
inactivated pathogens, split pathogens or specific parts of th e pathogen that may
induce a specific immune response (for example surface proteins or polysaccharide
toxins produced by the pathogen). By producing antibodies and memory cells
against the vaccine, the pathogen is rapidly cleared from the body before any
harm is done in case of an infection. As shown in
Tables 1 and 2, many different
vaccines are given at different time points in life.
In standard vaccination programs, all of the vaccines listed in
Tables 1 and 2 are
given as an intramuscular or subcutaneous injection, except for the polio vaccine,
which is also administered orally as a live attenuated virus vaccine. Much knowledge
has been obtained on parenteral vaccination over the past decades and injection of a
vaccine has some advantages over other route s of administration. One of the key
advantages is that after injection, it is certain that the antigen is administered in
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the intended dose to the vaccinee. Furthermore, the adminis-
tration time is quite short, which is favorable for mass vaccina-
tion programs or campaigns. Although injection of vaccines
has been the golden standard for almost one and a half
century, this route of administration has some major
drawbacks as well, mainly related to the use of a needle:
.
Administration of vaccines by injection requires trained
healthcare workers.
.
Re-use of needles or accidental needle stick injuries may
result in transmission of pathogens like the hepatitis B
virus and HIV.
.
The use of needles leads to needle waste.
.
Some people may refuse to be vaccinated due to their
needle phobia.
Other disadvantages, for example related to the vaccine
itself and the site at which the vaccine is delivered (under
the skin or in the muscle), may be:
.
The need for a cold chain, due to the limited stability of
the solutions for inje ction.
.
In the case of vaccine powders for reconsti tution, clean,
disinfected water is required.
.
The limited immunogenicity after injection of some
vaccines
[1,2].
Needle-free vaccine delivery could overcome s ome of the
disadvantages related to the parenteral route of administra-
tion. As an alternative to percutaneous administration,
administrationstothenose,mouth,lungsandskinhave
been studied as needle -free deliver y routes for va ccines
[3].
Especially the lungs are interesting for the delivery of
vaccines, since there is a large surface area available for inter-
action between the immune system and the vaccine.
Immune cells like alveolar macrophages and dendritic cells
are located throughout the lungs to capture and process for-
eign matter. After capturing, the antigen presenting cells can
traffic to draining lymph nodes where interaction with
T cells leads to the production of memory cells and specific
antibodies
[4].
There are sev eral viral and bacterial pathogens that are
transmitted through the inhalation of airborne particles.
Therefore, the lungs can be considered as a ‘natura l route
for vaccination aga inst these pathogens. Inhalation of the vac-
cine allows for dire ct delivery of the a ntigen to the port of
entry for these pa thogens. By inducing an immune response
at this site, individuals might be better protected against
inva ding pathogens than after parenteral administration, as
has been found for influenza
[5]. By presenting vaccines to
the mucosal site of the lungs, not only systemic IgG antibod-
ies might be produced, but in potential also local antibodies
in the form of secretory IgAs (s IgAs). The se mucosal immune
responses may give a broader protection (e.g., against drifted
vari ants) than the sing le IgG antibodies induced after paren-
teral vaccine administration. Various vacci nes are currently
being investigated for pulmonary administration, of which
a summary is given in
Table 3.Mostoftheresearchisstill
in the preclinical phases of development, but a few vaccines
have been tested in clinical trials. The most extensively stud-
ied vaccine for pulmon ary administration in humans is that
against measles
[6-12]. Measles is caused by a virus that is
transmitted through air borne particles. From the early
1960s until today, a number of pulmonary vaccination stud-
ies of the measles vaccine have been performed in children,
which show ed that an immune re sponse can be induced after
pulmonary administration of the vaccine. In most of these
studies a serocon versi on rate of 80% or higher was achiev ed.
This rate of seroconversion was also seen in children younger
than 9 months of age
[13,14]. In all of these studies, the vaccine
was delivered to the lu ngs by nebulization. On e follow-
up study, 6 yea rs after re-vaccination, showed that by pulmo-
nary administration of the measles vaccine, a longer lasting
anti body response w as achieved compared to injection
[8].
Another vac cine of which pulmonar y admin istrat ion has
been studied in ma n is influenza. About 40-years ago several
successful studies with nebulized in fluenza vaccine were
published
[15-17]. In one of these studies, cross-protection
against different variants of the virus was also found
[17].
It is believed that the different mucosal surfaces throughout
the body are linked together as a ‘common mucosal immune
system’ through the muscosal-associated lymphoid tissue
(MALT). Administration of vaccines to one mucosal site can
induce antibody production at another mucosal site
[18].
Broncho-associated lymphoid tissue (BALT), which is located
in the lung, is also a part of MALT. Therefore, local pro-
duction of antibodies at BALT can also induce local antibody
production at distant mucosal sites such as the nose (NALT),
Article highlights.
.
Pulmonary vaccination requires a suitable delivery
device, as well as a potent and stable vaccine
formulation.
.
A disposable, cheap, yet effective DPI suitable for the
target population appears to be the most optimal device
for pulmonary vaccination.
.
Jet nebulizers have been proven successful for vaccine
administration in clinical trials, but the necessity for a
pressurized (clean) air system limits their applicability in
mass vaccination programs.
.
For infants, the most important target group for
vaccination, pulmonary vaccination may be less suitable.
.
Techniques that produce powder formulations for
pulmonary administration in a one-step process reduce
the risks of contaminations and batch-to-batch
differences, as well as production costs.
.
Aqueous vaccine formulations are easy to manufacture
and have been proven successful in clinical studies, but
are less favorable due to stability concerns.
This box summarizes key points contained in the article.
W. F. Tonnis et al.
1384 Expert Opin. Drug Deliv. (2013) 10(10)
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gut (GALT), rectum (RALT) or vagina (VALT), yielding pro-
tection at other mucosal ports of entry as well. For example, in
a study in which the HPV vaccine was administered to the
lungs, locally produced antibodies were also found in the
female genital tract of the vaccinees
[19], which might give a
better protection at the mucosal surface that is the port of
entry for the pathogen.
Lastly, also for diseases that are neither transmitted via the
lung nor via other mucosal surfaces, pulmonary administra-
tion can be beneficial since it is a needle-free route of delivery.
Although the pulmonary route seems to exhibit various
advantages over administration by injection, some disadvan-
tages should be considered as well, the most apparent being
related to the delivery device:
.
There is still a desire for the presence of trained healthcare
workers, to instruct on the use of the inhalation device
and inhalation maneuver, although this may be less
critical compared to injection, since the administration
can be performed by the vaccinee.
.
A flaw in the inhalation maneuver may lead to ques-
tions whether the antigen is truly and completely
administered.
.
The safety aspects of this route of administration are not
well known. Local irritation of the lung can lead to
severe side effects or limit the number of vaccinations
that can be administered in this way
[20].
.
Pulmonary administrati on of vaccines may induce
inflammatory responses or exacerbation in patients with
lung-associated diseases, like asthma.
A prerequisite for an y compound to be administered
through the pulmonary route is that sufficient lung deposition
is achieved after the administration. The scientific back-
ground of lung deposition in relation to parameters such as
aerodynamic particle size in the aerosol, inhalation flow rate
or breath hold could easily fill this review. However, that is
beyond the scope of this review and for these aspects, refer-
ence is made to previous publications
[21,22]. For this review,
it is considered sufficient to assume that effective total lung
deposition occurs when sufficient aerosol is generated and
delivered in the appropriate aerodynamic size range with a
suitable (moderate) airflow rate.
It is clear that many challenges have to be overcome
before pulmon ary administration of vaccines can be widely
applied. Still, this route of ad ministration holds many
promises. This review focuses on the delivery de vices that
may be suitable for the administration of vaccines to the
lungs and on the formulation strategies for the vaccines
intended for pulmonary administration. A further aim is
to disc uss the critical parameters , such as tar get population
and formulation options, to help assessing whether pulmo-
nary administration of a specific vaccine may be feasible
and beneficial or not.
Table 1. Routine and non-routine vaccination per age group according to WHO guidelines [47].
Infants
(< 1 year)
Children
(1 -- 10 years)
Adolescents
(10 -- 18 years)
Adults
(18 -- 65 years)
Elderly
(> 65 years)
BCG -- -- -- --
-- -- HPV -- --
Hepatitis B -- Hepatitis B* Hepatitis B* --
Polio -- -- -- --
DTP DTP DTP -- --
Hib -- -- -- --
Pneumococcal -- -- -- --
Rotavirus -- -- -- --
Measles -- -- -- --
Mumps -- -- -- --
Rubella -- Rubella
z
Rubella
z
--
-- Influenza
§
Influenza
§
Influenza
§
Influenza
§
*Given to health care workers.
z
Given to adolescent girls and/or women of child-bearing age.
§
Given annually to persons at risk.
-- : Not given routinely to age group; BCG: Bacillus Calmette-Gue
´
rin; DTP: Diphteria, Tetanus and Pertussis; Hib: Haemophilus influenzae type b;
HPV: Human Papillomavirus.
Table 2. Non-routine vaccinations [47].
Vaccine Revaccination, years after
last vaccination
DTP 10 years
Hepatitis A 1 year
25 years after second
Yellow fever 10 years
Rabies 2 years*
Typhoid 3 years
*Also immediately after suspicious (dog) bite.
DTP: Diphteria, Tetanus and Pertussis.
Devices and formulations for pulmonary vaccination
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Table 3. Pulmonary vaccine formulations studied in (pre)clinical research.
Preclinical research
Pathogen/toxin Vaccine/strain Formulation Drying method Administration Animal Refs.
Corynebacterium diphtheriae CRM-197 PLGA nanoparticles
containing the antigen
Emulsion-based method As powder using an insufflator Guinea pigs
[81]
Corynebacterium diphtheriae Diphteria toxoid Microparticles of chitosan
and antigen
Super critical fluid drying As powder using an insufflator Guinea pigs [54]
Cytomegalovirus Glycoprotein B Liquid of antigen in
ISCOMATRIX
- As liquid via a bronchoscope Sheep [82]
Hepatitis B virus HBsAg PLGA/PEG nanoparticles
containing the antigen
Emulsion-based method
and spray drying
As powder by insufflator Guinea pigs [83]
Hepatitis B virus HBsAg PLGA microparticles
containing the antigen
Emulsion-based method As liquid: particles dispersed
into PBS
Rats [74,75]
Hepatitis B virus HBsAg PLA/PLGA nanoparticles
containing the antigen
Emulsion-based method As liquid: particles dispersed
into PBS
Rats [73]
Influenza virus Whole inactivated virus
and split-subunit vaccine
Lipid microparticles of
antigen
Spray drying As powder using an insufflator Mice [5]
Influenza virus Whole inactivated virus
and subunit vaccine
Liquid antigen suspension - As liquid using a micropipette
and microsyringe
Mice [84]
Influenza virus Subunit vaccine Powder of inulin and
antigen
Spray-freeze drying As powder using an insufflator Mice [85]
Influenza virus Subunit vaccine Powder of inulin and
antigen
Spray drying As powder using an insufflator Mice [25]
Influenza virus Subunit vaccine Liquid of antigen in
ISCOMATRIX
- As liquid via a bronchoscope Sheep [82,86]
Measles virus Edmonston-Zagreb vaccine Liquid - As liquid using a jet nebulizer
with or without face mask or
an ultrasonic nebulizer with a
paper cone
Monkeys [87]
Measles virus Edmonston-Zagreb vaccine Powder of trehalose and
antigen
Spray drying As powder using an insufflator Monkeys [88]
Measles virus Edmonston-Zagreb vaccine Powder of myo-inositol
and antigen
Super critical fluid
drying
As powder using the PuffHaler Cotton rats [89]
Measles virus Edmonston-Zagreb vaccine Powder of myo-inositol
and antigen
Super critical fluid
drying
As powder using PuffHaler or
BD Solovent
Monkeys [90]
Mycobacterium tuberculosis Ag85B vaccine Conjugate of antigen and
nanoparticle
- As liquid: conjugate dispersed
in PBS
Mice [72]
Mycobacterium tuberculosis Live attenuated BCG Powder of L-leucine and
antigen
Spray drying As powder using an insufflator Guinea pigs [51]
Mycobacterium tuberculosis Ag85B vaccine PLGA microparticles
containing the antigen
Spray drying As powder by insufflator Guinea pigs [76]
23-PPV: 23-valent pneumococcal polysaccharide vaccine; Ag85: Antigen 85; BCG: bacille Calmette-Gue
´
rin; CRM-197: Cross-reacting material-197; HBsAg: Hepatitis B surface antigen;
HPV16: Humanpapilloma virus 16; MMR: Measles, Mumps, Rubella; MVA85A: Modified vaccinia virus Ankara expressing antigen 85A; PBS: Phosphate buffered saline; PEG: Polyethylene glycol;
PLA: Polylactide; PLGA: Poly(lactic-co-glycolic acid); SEB: Staphylococcol enterotoxin B; VEE: Venezuelan equine encephalomyelitis.
W. F. Tonnis et al.
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Table 3. Pulmonary vaccine formulations studied in (pre)clinical research (continued).
Preclinical research
Pathogen/toxin Vaccine/strain Formulation Drying method Administration Animal Refs.
Mycobacterium tuberculosis MVA85A Suspension of antigen in
PBS
- As liquid using a vibrating
mesh nebulizer
Monkeys
[91]
Mycobacterium tuberculosis Ag85A vaccine Liquid - Intratracheal installation of
liquid formulation
Mice [92]
Ricin Ricin toxoid Suspension of antigen
encapsulated in liposomes
- As liquid using a syringe Rats [69,70]
Staphylococcol enterotoxin B SEB toxoid Liquid of antigen in
proteosome
- As liquid via endotracheal tube Monkeys [93]
VEE virus Live attenuated PLGA microparticle
containing the antigen
Emulsion-based method As liquid: microparticles
suspended in PBS
Mice [94]
Yersinia pestis F1 and V subunit vaccine PLA microparticles
containing the antigens
Emulsion-based method As liquid: microparticles
suspended in PBS
Mice [95]
Clinical research
Pathogen Vaccine Formulation Drying method Administration Subjects Refs.
Human papillomavirus HPV16 Suspension of antigen in
saline
- As liquid using a jet
nebulizer
Female
adults
[19]
Influenza virus Live attenuated virus Liquid - As liquid using an atomizer Adults [15]
Measles virus Edmonston-Zagreb vaccine Reconstituted lyophilized
vaccine
- As liquid using a jet
nebulizer and paper cone
as face mask
Children [6,9,11,12]
Measles virus Edmonston-Zagreb vaccine Powder myo-inositol,
sorbitol and antigen
Super critical fluid drying As powder using PuffHaler or
BD Solovent
Male adults [23,55]
Measles/Mumps/Rubella MMR vaccine Liquid - As liquid using an ultrasonic
nebulizer and paper cone as
face mask
Adults [7]
Mycobacterium tuberculosis BCG vaccine Suspension of antigen - As liquid: children were placed
in a chamber in which the
vaccine was aerosolized
Children [63]
Streptococcus pneumoniae 23-PPV Suspension of antigen in
saline
- As liquid using an jet nebulizer Adults [62]
23-PPV: 23-valent pneumococcal polysaccharide vaccine; Ag85: Antigen 85; BCG: bacille Calmette-Gue
´
rin; CRM-197: Cross-reacting material-197; HBsAg: Hepatitis B surface antigen;
HPV16: Humanpapilloma virus 16; MMR: Measles, Mumps, Rubella; MVA85A: Modified vaccinia virus Ankara expressing antigen 85A; PBS: Phosphate buffered saline; PEG: Polyethylene glycol;
PLA: Polylactide; PLGA: Poly(lactic-co-glycolic acid); SEB: Staphylococcol enterotoxin B; VEE: Venezuelan equine encephalomyelitis.
Devices and formulations for pulmonary vaccination
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2. Inhalation devices
As for the administration of any other substance to the lungs,
a delivery device that generates an appropriate aerosol for
inhalation is also required for pulmonary vaccination. Tradi-
tionally, three types of delivery devices can be distinguished,
which have been developed mainly for the administration of
locally acting drugs that are used in pulmonary diseases such
as asthma, COP D or cystic fibrosis. These three types are
dry powder inhalers (DPIs), (jet, ultrasonic and soft mist)
nebulizers and pressurized metered dose inhalers (pMDIs).
Inhalation devices can be divided into single-dose and
multi-dose, or into disposable (single-use) and re-usable
inhalers. Vaccines are usually given only once in a prolonged
period of time (years). Even booster doses are generally given
3 to 6 months after the initial vaccination. This makes
re-usable devices less suitable for pulmonary vaccination.
Therefore, pMDIs and many of the currently marketed
DPIs (e.g., Turbuhaler (AstraZeneca), Diskus (GlaxoSmithK-
line), Novolizer (MEDA)) are considered sub-optimal and
will not be further discussed in this review. Re-usable devices
that are loaded with a single dose, such as capsule-based DPIs
and nebulizers, seem better suited for vaccination. However,
the use of one device for more than one person may introduce
risks related to inadequate cleaning and disi nfection of the
device, such as the transmission of infectious diseases. Instead,
a disposable (single-use) device may be considered as the most
optimal choic e for pulmonary vaccination purposes. With
such a device, the risk of transmitting diseases through the
device is abated. However, the use of disposable devices may
dramatically increase the costs if such devices are expensive
to manufacture. Therefore, the most opt imal device for vacci-
nation is not only disposable, but also cheap and yet effective
of design. Moreover, the device should be simple to use and
robust in performance, since effective targeting with a single
dose is the objective, and good instructions for use by well-
trained healthcare workers may not always be availa ble.
This, in combination with cost aspects, excludes sophisticated
and expensive soft mist nebulizers from use for vaccine
delivery.
The two types of pulmonary delivery devices that would
allow for use in pulmonary vaccination thus seem single-
dose DPIs and nebulizers (
Figure 1). The choice for one of
these types of devices is on the one hand dependent on the
dose and formulation options of the active compound, as
described in the next section (Formulations). On the other
hand, the target population may play an important role too
in determining which device is preferred. Both types of devices
have some advantages and disadvantages for use in pulmonary
vaccination, which are discussed in the next paragraphs.
2.1 DPIs
DPIs have to contain the vaccine in a powder formulation
that has physicochemical properties that are compatible with
the design of the device, and that enable the conversion of
the highest possible powder fraction into an aerosol with the
appropriate ae rodynamic particle size distribution, generally
defined as 1 -- 5 µm. Most of the DPIs are passive devices,
Single-dose DPIDevice
Disposable
option available
Challenges
Formulation
Nebulizer
Yes, but a pressurized air
system is required
Formulation stability
(solution/dispersion)
or
Clean, disinfected water
required (reconstitution)
Long administration time
Large residual volume
(formulation losses)
Still not optimal for youngest
age group(s)
Aqueous solution/dispersion
or
Powder for reconstitution
Powder for inhalation
Ye s
Strict requirements for
powder formulation
Compatibility device and
formulation (combined
development is essential)
Production costs should be
as low as possible
Not suitable for youngest
age group(s)
Figure 1. Device and formulation options for pulmonary administration of vaccines.
W. F. Tonnis et al.
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which means that the person who inhales through the device
delivers the energy for dispersion of the formulation from
the inhalation flow that he or she gen erates. Therefore, no
external power source is needed, which incre ases flexibility
of use and reduces the production costs. DPI production
may be quite cheap in spite of an effective design, which
allows for the use of disposable devices. Since a DPI is oper-
ated with a single inhalation maneuver, the administration
time is short. However, this maneuver must be taught, under-
stood and particularly be achievable by the vaccinee before the
actual inhalation of the vaccine can take place. An incorrect
inhalation may result in insufficient dose of the vaccine reach-
ing the lungs. Another challenge in the use of DPIs is the set
of requirements for the powder formulation. First, the active
particles should have the proper (1 -- 5 µm) aerodynamic
size distribution for effective lung penetration, although the
most favorable size distribution may depend on the precise
target area and the velocity by which the particles are inhaled.
Second, these powders should be dispersed well into an aero-
sol of primary particles by the inhaler at attainable inspiratory
flow rates through the device and within the inhaled volume
of air. Various characteristics of the powder formulation,
such as a high cohesiveness, a high water content or extreme
compactibility, may negatively influence dispersion behavior
as well as inhaler retention.
Up to date, only one clinical trial has been described that
uses DPI technology to deliver a vaccine (results not pub-
lished yet)
[23]. In this trial, two devices are being examined
for administration of dry powder measles vaccine, the
Puffhaler (Aktiv-Dry) and the Solovent (BD Technologies).
Both devices have been proven successful in a study with rhe-
sus macaques, in which a single dose of dry powder measles
vaccine induced durable, fully protective immunity
[24].In
contrast to most DPI s, both the Puffhaler and the Solovent
are active devices. The Puffhaler uses compressed air for dis-
persion of the inhalation powder. The powder is released
from a single-dose blister into an aerosol reservoir by squeez-
ing a bulb. The reservoir, equipped with facemask for children
or mouthpiece for adults, is then disconnected from the
device, after which the aerosol is ready for inhalation. The
Puffhaler is claimed to be suitable for both adults, who can
inhale the dose in a single-breath mane uver, and for children,
who may need multiple inhalations for the administration of
the full dose. The Solovent consists of a syringe, a capsule
containing the powder, a spacer and a facem ask. By pressing
the syringe, thin films covering the capsule disrupt and the
powder is released into the spacer, from which the dose can
be inhaled.
Promising preclinical data are also available for powder for-
mulations with influenza vaccine using the Twincer concept
for pulmonary delivery
[25]. The Twincer concept has a dis-
posable design, which was initially developed for the adminis-
tration of (highly dosed) antibiotics
[26-28], but the concept
may also be applicable for pulmonary vaccination. Further-
more, Manta Devices LCC has developed the Torus DPI,
a low-cost, passive DPI platform suitable for single- use
applications, such as vaccines
[29]. In that respect, also other
disposable DPIs that are currently being developed for other
applications may be interesting for pulmonary vaccination.
An extensive overview of disposable DPIs has been given by
Friebel and Steckel
[30], including the TwinCaps (Hovione),
the Conix One (3M) and the DirectHaler Pulmonary
(Direct-Haler A/S). Especially the latter could be an interest -
ing alternative for vaccination purposes because of its low
production costs.
2.2 Nebulizers
Nebulizers generate aerosols from aqueous solutions or
suspensions. Basically, two classic nebulizer types exist: ultra-
sonic and jet nebulizers. Ultrasonic nebulizers produce drop-
lets by applying high frequency pulses from an oscillating
piezo element to the drug solution. Such high frequency
pulses have been shown to induce protein inactivation and
aggregation
[31-34]. Since most antigens are of protein/peptide
nature or at least a component of them, ultrasonic nebulizers
are considered inapt for the nebulization of vaccines and are
not discussed further. In jet neb ulizers, a two-fluid nozzle is
used to produce the aerosol. The relatively wide size distribu-
tion of the droplets from such nozzles is adjusted to the
desired range of 1 -- 5 µm by removal of the largest droplets,
which is achieved by impaction against a baffle in the aerosol
stream. Many other variables can influence th e droplet size
distribution and the output rate of the nebulizer, including
the physicochemical properties of the drug solution
[35-39],
the jet pressure for the nozzle
[31,39-41] and the breathing pat-
tern of the patient
[42]. Jet nebulizers are frequently used for
the administration of drugs that do not have an indication,
registration or approval and they are often driven by compres-
sors (or air pressures from the mains) for which they were not
developed. This bears the risk that the nebulizers are operated
at incorrect jet pressures, resulting in rather ineffective and
uncontrolled therapies
[43]. Other major drawbacks of jet neb-
ulizers relevant to vaccine inhalation are the long administra-
tion time, which is a problem in mass vaccination programs,
and the large residual volume in the nebulizer cup, which
may result in a waste of vaccine amounting to 30% of the
total dose, thereby increasing costs.
Most nebulizers, while in many cases designed to be dispos-
able, are re-usable devices. This has the consequence that they
need to be cleaned and disinfected on a regular basis, which
makes them less suitable for use by different patients. Although
various disposable (single-use) jet nebulizers are available as
well, they may not all be as effective as re-usable devices with
respect to droplet size distribution and output rate
[44],and
they still require a pressurized (clean) air system, which limits
their use to hospital-like environments.
Jet nebulizers exert high forces on the liquid during the
aerosol generation too, especially because the solution is
recycled many times during the aerosolization process before
it is inhaled. This may result in degradation of the product,
Devices and formulations for pulmonary vaccination
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especially when versatile products like vaccines are adminis-
tered. Khatri et al. showed significant degradation of a model
protein during both jet and ultrasonic nebulization
[45]. Such
rapid degradation or structure loss, due to the energy input
from the aerosolization principle, could significantly reduce
the antigenicity of the vaccine.
2.3 Applicability of inhalation devices
The design of a proper delivery device may be the major chal-
lenge for pulmonary vaccination to become successful. The
device is preferably operated by the inhalation flow of the vac-
cinee. For this, it is important that the vaccinee is able to ful-
fill the requirements with respect to inspiratory flow rate,
inhaled volume and duration of the inhalation, to guarantee
that a sufficient mass fraction of the dose is deli vered to the
target area. The ability to perform a proper inhalation maneu-
ver is popula tion dependent, due to physical capacities and
proper understanding of the handling of the inhalation
device. Therefore, pulmonary administration might not be
feasible for all age groups. Especially in young children, pul-
monary administration may be less effective and reliable
than injection.
For its vaccination programs, the WHO distinguishes three
target groups: children, adolescents and adults
[46]. Especially
for children, many different vaccines are recommended for
routine vaccination. These vaccines should be administered
at various moments in life, ranging from as soon as possible
after birth up to the early teens (and continuing into adoles-
cence) (
Tables 1 and 2). The age or developmental stage of a
child largely determines whether pulmonary vaccination
may be feasible, so a furthe r division into dif ferent age groups
is required. The first group, infants, cannot be taught how to
inhale, so a device that is operating by the inhalation of the
subject (passive DPI) is not feasible in the youngest age group.
Furthermore, infants are obligatory nose breathers and they
have a high breathi ng frequency. These two characteristics of
the breathing pattern result in poor convective transportation
of the aerosol into the lungs, and thus in poor deposition. For
toddlers, no to poor understanding of the instructions is also a
limiting cognitive constraint. Therefore, a passive DPI is not
suitable for this age group either. Active devices equipped
with a facemask, such as the Puffhaler and Solovent, may be
applicable. However, they do not resolve the proble ms related
to the nose breathing and a poor breathing pattern. When
children become older they become aware of what is happen-
ing to them. Toddlers have a tendency to be afraid of
unknown people and situations, which may result in a poor
cooperation. In a study with 94 infants and young children
(under 5 years of age), about 30% of the children did not
accept a facemask on first use
[47]. Furthermore, it has been
shown that struggling of toddlers during pulmonary adminis-
tration (using a pMDI and facemask) results in a very low
(below 10%) and highly variable lung deposition
[48]. Chil-
dren older than 4 -- 6 years may be able to understand the
inhalation instructions, and their inquisitive nature may
help to make them co-operative. However, their smaller
lung volumes compared to adults should be considered. Espe-
cially in preschool children, inspiratory vital capacities (IVC)
may still be insu fficient for complete dose release from a (pas-
sive) DPI. Therefore, development of special devices for these
children may be necessary. Adolescents and adults generally
have no constraints regarding pulmonary vaccination, in
contrast to elderly -- another group not distinguished in the
WHO guidelines -- who may exhibit physical limitations.
However, the potential problems in this target group are of
a different order than those in infants and toddlers, and are
limited to only a part of this population.
Table 4 gives an overview of all age groups and the physical,
cognitive and emotional aspects that should be considered for
pulmonary vaccination, as well as the inhalation devices that
Table 4. Target groups for pulmonary vaccination and the aspects to be considered for use.
Age (years) Physical Cognitive Emotional Pulmonary devices
Infants 0 -- 1 Nose breathing,
IVC
No understanding None (Nebulizer with face mask)
Toddlers 1 -- 3 IVC Insufficient
understanding
Fear of unknown
situations/people
(DPI)
Nebulizer with face mask
Preschool children 3 -- 6 IVC Poor understanding Fear of needles, inquisitive (DPI*)
Nebulizer (with face mask)
School children 6 -- 12 (IVC) Partly poor
understanding
Fear of needles, like
to be involved/in control,
inquisitive
DPI
Nebulizer
Adolescents 12 -- 18 None Mostly none Fear of needles DPI
Nebulizer
Adults 18 -- 65 None Mostly none Fear of needles DPI
Nebulizer
Elderly > 65 IVC Mostly none Fear of needles DPI
Nebulizer
*Instead of a passive dry powder inhaler, a simple (possibly re-usable) active device with disposable aerosol chamber may be used.
IVC: Inspiratory vital capacity.
W. F. Tonnis et al.
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may be applicable. In addition to all the constraints, needle
phobia is an argument in favor of pulmonary administration,
which should be considered as well.
3. Formulations
Most currently used vaccine formulations are unstable. To
slow down the degradation processes, a cold chain is applied,
which is challenging and expe nsive and par ticularly in devel-
oping countries often not possible. Therefore, it would be
highly beneficial for storage and distribution around the
world if the stability of the vaccine could be improved to
such an extent that the cold chain would be superfluous.
This does not only apply to parenteral formulations, but to
pulmonary formulations as well.
As described in the section on devices, vaccines can be
delivered to the lungs in for m of powders using a DPI or as
aqueous solutions by a nebulizer. In this section, the pre-
paration methods, the stability of the formulations, and the
advantages and disadvantages of both types of formulations
are discussed.
3.1 Powder formulations
To our knowledge, no large clinical studies using powder for-
mulations for pulmonary administration of vaccines have yet
been published. However, in a current clinical trial on the
pulmonary delivery of measles vaccine, a powdered vaccine
is being used
[23]. Other examples of formulations discussed
in this section have only been tested in vitro or preclinically.
3.1.1 Methods for preparing powder suitable for
pulmonary delivery
Vaccines are usually produced as an aqueous solution or aque-
ous dispersion. From these formulations, powders can be pre-
pared by various drying techniques, such as spray drying,
freeze drying, spray-freeze drying, vacuum drying or supercrit-
ical fluid drying
[49,50]. As mention ed before, the primary par-
ticle size should be in the correct aerodynamic size range
(1 -- 5 µm) in order to be able to penetrate into the desired
area of the lungs. Therefore, the technique that is used to pre-
pare the powder formulation should ideally yield particles in
this size range in order to avoid further processing. Techni-
ques like spray drying and spray-freeze drying disperse an
aqueous solution of the vaccine into small droplets, which
are dried by heat (spray dryi ng) or by freezing an d subsequent
sublimation of water (spray-freeze drying), respectively. By
choosing the appropriate concent ration of the solution and
size of the droplets, a powder can be obtained consisting of
particles with the desired aerodynamic size in a one-step
process. Both spray drying and spray-fr eeze drying have
been successfully used to prepare powder formulations of
various vaccine in the correct size range for pulmonary
administration
[25,51,52].
Another method that is used to prepare powder formula-
tions of vaccines in a one-step process is by supercritical
fluid drying
[53,54]. This technique uses supercritical carbon
dioxide to disperse an aqueous solution of the antigen, which
is subsequently dried by a hot carrier gas. A powder formula-
tion of the measles vaccine was obtained by usage of the
Carbon dioxide Assisted Nebulization with the Bubble Dryer
(CAN-BD) yielding particles with an aerodynamic size of
3 -- 5 µm
[53]. This powder is currently under investigation
in the previously mentioned clinical trial
[55].
With techniques lik e freeze drying and vacuum drying, a
material is obtained that does not consist of particles with a
defined size distribution. Therefore, a second process is
required for obtaining the correct particle size. This may for
example be milling or slugging
[56]. However, this multiple-
step processing can induce additional stress to the antigen
on top of the drying process itself. Furthermore, the risks of
contaminations and batch-to-batch differences, as well as
production costs increase with every additional process step.
3.1.2 Stability of the vaccine in powder formulations
Vaccines are often composed of complex structures, which
imply that they are usually unstable. Depending on the drying
technique, the vaccine will be exposed to certain stresses,
which could damage the vaccine, and thus deteriorate its
antigenicity. The various stresses that may occur during
drying have been reviewed by Amorij et al.
[49].
To prevent degradation during processing, stabilizing
excipients such as polyols and sugars (e.g., inulin, trehalose,
myo-inositol, sorbitol) can be used
[25,53,57-60]. Depending
on the type of dryi ng process, various theories have been
described to explain the stabilizing action of sugars. However,
the so-called water replacement theory applies to all drying
techniques. During drying, the hydration shell around the
antigen is removed, which can result in degradation
[60].
According to the water replacem ent theory, the hydrogen
bonds between water molecules and the vaccine (the hydra-
tion shell) are gradually substituted by hydrogen bonds
between the hydroxyl groups of the sugar during drying
[60].
Thus, the hydration shell is replaced by an embedding in
sugar molecules. By this water replacement, dehydration
stresses can be avoided, and thus also degradation processes
associated with these stresses. To efficiently form hydrogen
bonds, the sugar molecules should accommodate the irregular
structure of the vaccine, which implies that the sugar should
be in the amorphous and not in the crystalline state. Further-
more, incorporation of the individual vaccine molecules or
particles in sugar also prevents interaction between these
species, by which for example aggregation is prevented.
Once in the dry state, the sugar matrix can also provide
excellent storage stability of the vaccine by a mechanism,
which is referred to as vitrification
[61]. Such excellent storage
stability is reached when the sugar matrix is in the glass state.
By incorporating the vaccine in such a glassy matrix, the trans-
lational molecular mobility is strongly reduced. This reduced
mobility will prevent or strongly slow down degradation
routes which require molecular mobility, like denaturation.
Devices and formulations for pulmonary vaccination
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To stabilize the vaccine during storage, the matrix should
remain in the glass state. Therefore, storage should be at a
temperature below the transition temperature (T
g
). Above
this temperature, the glass becomes a rubber, which can crys-
tallize. During crystallization, the hydrogen bonds between
the sugar and the vaccine are broken and the stabilizing action
of the sugar is lost. Therefore, sugars with a high T
g
should be
used. Next to the high T
g
, the stabilizer should also be cheap
and readily available. Furthermore, if the vaccine is a protein,
the sugar should not contain reduci ng groups. Reducing
groups can react with amine groups of the protein to form a
Schiff’s base, the first step of the Maillard browning leading
to deg radation of the vaccine. Finally, the stabilizers should
be well soluble in water and well tolerated after pulmonary
administration. Trehalose and inulin both comply with
these requirements.
In summary, having the vaccine in the dry and stable state
can have tremendous advantages for storage and distribution .
For some vaccines it has already been shown that it is possible
to store them at ambient or even elevated temperatures with-
out deterioration for extended periods of time by incorporat-
ing them in a sugar glass matrix, for example the subunit
influenza vaccine (using inulin) and a novel tuberculosis
vaccine (mannitol-based stabilizers)
[25,52].
A final advantage of using excipients for powder formula-
tions is that they can act as bulking agent. Vaccines are often
administrated in doses in the microgram range, which is too
small to be administered reproducibly by inhalation. When
bulking agents are used, the total amount of powder to be
inhaled is increased, which makes accurate dosing possible.
3.2 Liquid formulations
The majority of all formulations used in clinical studies of
pulmonary vaccine delivery so far, were aqueous solutions or
dispersions administered by nebulization
[6,7,9-12,15-17,19,62,63],
except for the powdered measles vaccine discussed earlier.
The majority of the studies in which the vaccine was nebu-
lized demonstrated that an immune response could be
induced by pulmonary administration, showing that this
technique can be an effective way of administrating vaccines
to the lungs.
3.2.1 Preparation of liquid formulations for
nebulization
The main reason for using nebulization in clinical studies so
far seems the ease of formulating, which is far less complicated
and time consuming than preparing a powder formulation for
inhalation. The sole requirement for a wet formulation is that
the pH is between 3 and 8.5, according to the European Phar-
macopoeia
[64]. Ideally, the formulation is also sterile and
(approximately) isotonic, because hypo- and hypertonic solu-
tions can induce bronchoconstriction in susceptible sub-
jects
[65,66]. Since the droplets are formed by the nebulizer
during administration, there is no requirement for the particle
size distribution of the formulation itself. Therefore, the
vaccine should only be diluted to the right concentration
and be at an optimal pH for both the stability of the vaccine
and require ments according to the pharmacopeia.
3.2.2 Stability of the vaccine in liquid formulations
During aerosolization into small droplets, the vaccine might
experience shear stresses. In many of the pulmonary vaccin a-
tion studies in children, the solution is placed on crushed
ice to keep the solution cool during nebulization. This can
lead to freezing stress when the solution is (partly) frozen.
Even though the preparation of the aqueous formulation is
rather easy, the administration itself may damage the vaccine.
To prevent degradation of the vaccine in aqueous solutions,
stabilizing excipients can be added. It is well known that com-
patible osmolytes (e.g.,
L-serine, glycine, trimethylamine
N-oxide) and surfacta nts (e.g., Tween 80) can stabilize bio-
pharmaceuticals in an aqueous environment. The stabilizing
action of compatible osmolytes have been descri bed to the
preferential hydration theory
[67]. This theory has been pro-
posed for the stabilization of proteins but nowadays it is also
applied for other biopharmaceuticals. Preferential hydration
means that the osmolyte is excluded from the surface of the
protein. By the preferential exclusion, the Gibb’s free energy
of the protein is increased. The surface area for exclusion
and therefore the magnitude of the increase in Gibb’s free
energy is greater for the denatured state than for the native
state. As a consequence, the Gibb’s energy of denaturation is
increased and the native state is stabilized
[68]. Surfactants
can be used to prevent protein aggregation by preventing
adsorption to air/water and solid/water interfaces. Surface
tension forces at these interfaces can change the conformation
of the biopharmaceuticals, which leads to aggregation
[68].
Surfactants prevent th is by adsorbing to these interfaces
themselves.
Stabilizing excipients can be used to stabilize the vaccine
during administration but can also have a positive effect on
the storage stability. Another way to improve the storage
stability is by preparing stable powder formulations, which
can be reconstituted prior to nebulization. In this case, the
particle size of the powder is not importa nt, as is the case
for powders that are inhaled as such. To this end, also other
drying techniqu es then spray dryi ng and spray-freeze drying
can be used since no particle size requirements exist. This
strategy -- reconstitution of a powder formulation -- has been
successfully applied in large clinical studies with the measles
vaccine in Mexican school children. Here, the vaccine was
lyophilized and reconstituted prior to nebulization by adding
a diluent
[6].
3.3 Current formulation developments
In the last dec ades, novel types of formulations like lipo-
somes
[69,70], nanoparticles [71-73] and microparticles [54,74-76]
have been developed for the delivery of antigens to the lungs.
The advantage of particulate delivery systems is that they are
well recognized by antigen presenting cells compared to
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soluble antigens. Furthermore, these systems can be equipped
with immunostimulating molecules (e.g., Toll-like receptor
ligands) or specific antibodies that bind to the antigen pre-
senting cell (e.g., anti-DEC205
[77]) in order to further
improve the immune response after pulmonary administra-
tion. All of these types of formulations are still in the pre-
clinical phase and can either be administered to the lungs as
a powder or dispersed in a liquid aerosol.
4. Conclusion
For pulmonary vaccination to be successful, two key aspects
have to be considered. Besides a potent and stable formula-
tion, a suitable delivery device is of utmost importance.
Such a delivery device should not only disperse the vaccine
effectively into the particles within the respirable size range,
but it should also be easy to use by the target population
and cheap to manufacture if intended for use in mass vaccina-
tion programs. Basically two types of pulmonary delivery
devices seem appropriate for vaccination: DPIs and jet nebu-
lizers. A disposable, cheap, yet effective DPI suitable for the
target population may be the most optimal device for pulmo-
nary vaccination. For infants, the most important target
group for vaccination, pulmonary vaccination may not be fea-
sible at all. Any vaccine formulation should be compatible
with the delivery device. An inhalation powder has much
stricter requirements than a solution or powder for reconstitu-
tion. Ideally, the powder formulation is manufactured in a
one-step process to reduce costs and the risk of contamination
and batch-to-batch differences. Only if the vaccine can be for-
mulated in such a way that powder particles suitable for inha-
lation are obtained, a DPI can be used. Aqueous vaccine
formulations may be easier to develop, but here chemical
and microbial stability remains a key issue.
5. Expert opinion
Pulmonary vaccination is a promising but challenging alterna-
tive to vaccination by injection. The most apparent promises
are the reduced use of needles and the additional mucosal
immunity that may be obtained. The most challenging aspect
of pulmonary vaccination is the delivery device, with regard to
both its design and applicability in the target population. Pul-
monary vaccines against various pathogens have been proven
successful in various phases of (pre)clinical research, but this
alternative vaccination route is still not used at this moment.
Obviously, further clinical development and market introduc-
tion are difficult hurdles to take. The main difficulty for trans-
lating a vaccine that is successful in a small-scale study setup
into one that is suitable for mass vaccination programs is the
necessity of a combined approach of device and formulation
development into a system that can effectively be used by
the target population. In most clinical studies so far, the vac-
cine was delivered to the lungs by nebulization, proving the
feasibility of pulmonary vaccination. However, we believe
that nebulization cannot be the standard for routine vaccina-
tion programs, due to the costs, the duration of the nebuli-
zation process and the necessity of a pressurized air system.
Stability of the vaccine is another problem inherent to
nebulization. A powder formulation for reconstitution may
help to solve this problem, but this requires clean, disinfected
water. In some remote areas of the world, this may be
problematic, thereby introducing the risk for microbial
contamination.
Administration of the vaccine as a powder would take away
most issues concerned with microbial contamination and sta-
bility. Techniques like spray drying, spray-freeze drying and
supercritical fluid drying have the advantage of obtaining a
powder formulation suitable for inhalation in a one-
step process. The requirements for an inhalation powder are
much stricter than for a powder for reconstitution and the
powder should be compatible with the design of the inhaler
to be used. A situation in which interesting formulations are
being developed that cannot be delivered properly is pointless.
Therefore, we believe that concurrent formulation and device
development is critical for the translation of clinical studies to
daily practice. In this respect, it should be noted that not just
the formulation has a strict set of requirements, but also the
inhaler. A DPI can only be used in mass vaccination programs
if it is cheap to produce, while effective too. Preferably, it
should be a disp osable device, to minimize the risk of trans-
mission of infectious diseases that would still be present if
the device is to be used by more than one person. Lastly,
but quite importantly, the device should be easy to operate
by the target population. It is obvious that a vaccination pro-
gram can never be successful if the vaccinees do not compre-
hend how to use the inhaler. Especially the older age groups
(adolescents, adults, elde rly) may be considered suitable target
populations for pulmonary vaccination . Therefore, it may be
an interesting approach for vaccination campaigns directed
towards these populations, for example catch-up vaccination
with DTP or vaccinations against HPV and influenza
[78,79].
One aspect that has not been discussed yet, but which we
consider highly relevant, is the optimal deposition site for vac-
cines. In the case of the measles vaccine, pre clinical studies
suggest that the peripheral lung is the best vaccination target,
but all methods that delivered the vaccine past the nose cavity
or mouth gave a sufficient immune response, which implies
that the exact deposition site may be of less importance
[80].
However, this has not bee n studied in humans yet and neither
for other vaccines. A systematic clinical trial in which the dif-
ferent lung regions are targeted could unravel the importance
of the deposition site regarding both the systemi c and mucosal
immune response. Data obtained with such a trial could pro-
vide the basis for formulation and device requirements for
pulmonary vaccination. If reaching the peripheral lungs is
not essential for a proper immune response, aspects like
proper inhalation technique and exact particle size are less
important, which would greatly increase the feasibility of
pulmonary vaccination.
Devices and formulations for pulmonary vaccination
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Acknowledgment
W F Tonnis and A J Lexmond equally contributed to
this work.
Declaration of interest
The authors state no conflict of interest and have received no
payment in preparation of this manuscript.
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